Density is the fundamental physical property that can be used alone or in conjunction with other properties to characterize fluids in many industrial processes, such as in the petroleum, chemical and food industries. Laboratory measurements of density can reach accuracies of 0.01% or less, if sufficient care is taken during the transportation and analysis of the fluid sample.
Various methods of measuring the density of a fluid have been proposed however, are not well suited for in situ use. For example, many conventional densitometers are limited by their physical construction if the acoustic transducer(s) are affixed directly to the fluid sample chamber. When the chamber is subjected to extreme in situ pressure, the chamber dimensions may be affected thus, requiring recalibration of the device with each change in the fluid sample pressure. The net effect is a greater degree of uncertainty in the density measurements taken. Various other density measurement devices have been developed to maximize precision while reducing material complexity as discussed in Vol. 49, No. 9, of the September 2002 issue of IEEE Transactions On Ultrasonics, Ferroelectrics, and Frequency Control, titled Ultrasonic Densitometer Using a Multiple Reflection Technique by Ricardo Tokio Higuti and Julio Cezar Adamowski. The accuracy of these devices, however, is also limited by temperature and pressure conditions of the fluid sample, which may alter the dimensions of the device. As a result, these devices may require excessive recalibration with each fluid sample and/or may present unacceptable results at extreme pressures.
Another common ultrasonic method used to measure the density of fluids is based on the measurement of the reflection coefficient at the interface between a reference material and the fluid sample as more fully described in U.S. Pat. Nos. 5,708,191 and 5,886,250. The '191 and '250 patents each describe methods for determining the density of a fluid sample by means of a material wedge positioned in the fluid. The material properties of the wedge are limited to materials having an acoustic impedance no greater than eleven (11) times that of the fluid sample. These methods are therefore, limited by their material requirements, which may be wholly inappropriate for certain fluids and at in situ conditions.
In the petroleum industry, reservoirs are usually several thousands of feet from the earth's surface and are typically under extreme pressures reaching several tens of thousands of pounds per square inch. Geothermal temperatures at these depths are on the order of 250° F. or more. Most conventional tools and associated methods are therefore, either inappropriate or impractical for taking density measurements of formation fluid samples from the earth at in situ temperatures and pressures. Consequently, formation fluid samples taken by conventional means, such as by a wireline device, are normally shipped to a laboratory where, under controlled conditions mimicking in situ pressure and temperature, density and other properties may be determined. The fluid properties may substantially impact decisions as to whether production may be economically achieved and, if so, the duration, expense and unit price of such production.
Transfer of the formation fluid sample to the surface environment, however, may induce several irreversible changes in the fluid sample. For example, during the rise of a fluid sample to the surface, both pressure and temperature drop substantially. Pressure and temperature changes may cause certain components of the fluid sample to irreversibly precipitate from solution and/or colloidal suspension, causing the fluid sample to be underestimated by surface testing. Production events such as paraffin or asphaltene deposition may also be avoided by preservation of the formation fluid sample at in situ conditions. For these reasons, preservation of the in situ state of a fluid sample during testing is preferred over mimicking in situ conditions.
One example of a conventional wireline sampling device that addresses this issue is illustrated in U.S. patent application Ser. No. 10/242,112, published on Apr. 10, 2003 and incorporated herein by reference. The '112 application describes a device or tool for maintaining the single phase integrity of a deep formation well sample that is removed to the surface for testing. Referring to FIG. 1 of the '112 application, the sampling and measuring instrument (tool) 13 is positioned within borehole 10 by winding or unwinding cable 12 from hoist 19, around which cable 12 is spooled. Depth information from depth indicator 20 is coupled to signal processor 21 and recorder 22 when instrument 13 is disposed adjacent an earth formation of interest. Electrical control signals from control circuits 23 are transmitted through electrical conductors contained within cable 12 to instrument 13. The sampling mechanism or tool 13 is comprised of a hydraulic power system 14, a fluid sample storage section 15, and a sampling mechanism section 16. Sampling mechanism 16 includes a selectively extensible well wall engaging pad member 17, a selectively extensible fluid admitting sampling probe member 18, and bi-directional pumping member 19. Within the sample storage section 15 are one or more sample accumulation chambers 30. FIG. 2 schematically illustrates a fundamental configuration of accumulation chamber 30. While improving on the preservation of in situ conditions of the fluid sample, this tool does not address other problems associated with analyzing the formation fluid sample at a lab, such as:
i) limitations on the available number of fluid samples using conventional wireline devices;
ii) transport delays;
iii) deterioration of fluid samples by improper handling and conditioning;
iv) delayed use of test results for field appraisal (hydrocarbon potential) and well planning;
v) limitations on lab conditions and instruments; and
vi) export restrictions.
Some fluid properties, however, may be analyzed in situ as illustrated in U.S. Pat. No. 6,683,681 B2, issued Jan. 27, 2004 and incorporated herein by reference. The '681 patent describes an apparatus and method for measuring the refractive index of fluids along a continuum, for measuring attenuated reflectance spectra, and for interpreting the measurements made with the apparatus to determine a variety of formation fluid parameters. This device, however, may require more complex and sophisticated equipment than is necessary or desired to determine certain physical parameters of a formation fluid sample-particularly acoustic velocity.
Other conventional techniques may propose an estimated or simulated pressure, volume and temperature (PVT) of the fluid sample based upon pressure gradients and geochemical parameters of the fluid sample in situ. Conventionally proposed index and/or estimate techniques may be limited, however, by the physical properties of the fluid sample that must be analyzed and their accuracy, which may depart as much as 10-15% from laboratory values.
There is, therefore, a need for a device capable of accurately determining fluid properties such as velocity, volume, density, compressibility and viscosity with nominal calibration at in situ conditions. Additionally, there is a need for a device that is simple, efficient, and easily incorporated into conventional wireline fluid sampling tools or any downhole sampling device. Finally, such a device should also be capable of analyzing similar fluid properties in other industries.